Porous, cellular material can be generally described as having either closed cells, in which the cells or pores are not interconnected, or open cells, in which the cells or pores are interconnected and may extend to the surface of the material in which they are formed and display the structure and appearance of open pits. The cellular fibers of the present invention predominantly contain the closed type of cells.
In the past, cell formation has been used in thermoplastic sheet materials using practices as described in U.S. Pat. No. 2,531,665; U.S. Pat. No. 2,751,627; U.S. Pat. No. 4,473,665; and U.S. Pat. No. 5,158,986 the teachings of which are all incorporated by reference as if fully set forth herein. However, the technology embodied in these patents which addresses cell formation in thermoplastic sheet materials and methods to reduce out-diffusion of an impregnating gas to increase nucleation is not believed to be adaptable to forming fibers.
In the past, cell formation has been achieved in fibers by dispersing blowing agents into the molten polymer prior to extrusion. A wide variety of agents has been used including air, nitrogen, chlorinated fluorocarbons, and other gases, as well as volatile materials that are gaseous at molten polymer temperatures, such as methylene chloride and other halogenated hydrocarbons, materials that decompose to form gas products (such as azides), and materials that react to form gaseous products, such as acids and carbonates. The blowing agent may be added to the precursor resin or dispersed into the molten polymer. For example, U.S. Pat. No. 4,164,603 and divisionally related U.S. Pat. No. 4,380,594 (both incorporated by reference herein) are directed to processes and fibers with variable cells formed using a silicon blowing agent. U.S. Pat. No. 4,728,472 (incorporated by reference) describes a process to produce fibers with closed cells that requires the introduction of a fluorocarbon blowing agent into a molten polymer. While it is possible to achieve a percentage of closed cells using a blowing agent in a fiber extrusion process, experience indicates that the process yields a material with an undesirably high closed cell length to diameter ratio (greater than 500 and up to 2,000). Moreover, such processes may produce undesired levels of open cells.
In actual practice there are two primary drawbacks to the process of simultaneously extruding and foaming fibers to generate a cellular structure. First, such practices give rise to enhanced manufacturing difficulty due to the complexity of the process. Second, such practices generally provide poor uniformity. In particular, when extruding foamed fibers it is extremely difficult to extrude small uniform fibers without breaking the filaments. The polymer filaments have lower tenacity making it difficult to draw them properly. The lower tenacity also makes it more difficult to properly texture the yarn, so it loses body and texture that is needed in the final fabric. Further, during yarn formation it is difficult to spin foamed fiber at the same rate and quality as non-foamed fiber. In addition, many of the additives used to improve production rate, such as silicon oil or polydimethylsiloxane are undesirable in the final fabric. Such additives can have such adverse effects as creating uneven dyeings, leaving deposits on the processing machinery, and increasing the flammability of the fabric. It is also believed to be difficult to controllably vary the level of opacity at different zones along the length of the fiber when foaming and extrusion are carried out simultaneously. The ability to provide such controlled variation may be desirable for some applications.
As regards the above-referenced problem of poor uniformity, it is not possible to control the shape, size and distribution of the cells during simultaneous foaming and extrusion. In particular, the closed cells of fibers formed from simultaneous extrusion and foaming have undesirably high length to diameter (L/D) ratios. More specifically, in such prior art fibers the cells nucleate coming out of the extrusion head and the L/D (length to diameter ratio) increases as the fiber is drawn down to the desired denier. Although the cells have a large volume, the number of cells per unit length is consequently small. Conversely, greater light scattering corresponding to enhanced opacity (which is desirable to enhance whiteness) is achieved by a larger number of cells per unit length and, therefore, a larger surface area.